1. Field of the Invention
Embodiments of the present invention generally relate to completion of a well. More specifically, embodiments of the present invention pertain to analysis of different drilling methods used for completing a well.
2. Description of the Related Art
Historically, wells have been drilled with a column of fluid in the wellbore designed to overcome any formation pressure encountered as the wellbore is formed. This “overbalanced condition” restricts the influx of formation fluids such as oil, gas or water into the wellbore. Typically, well control is maintained by using a drilling fluid with a predetermined density to keep the hydrostatic pressure of the drilling fluid higher than the formation pressure. As the wellbore is formed, drill cuttings and small particles or “fines” are created by the drilling operation. Formation damage may occur when the hydrostatic pressure forces the drilling fluid, drill cuttings and fines into the reservoir. Further, drilling fluid may flow into the formation at a rate where little or no fluid returns to the surface. This flow of fluid into the formation can cause the “fines” to line the walls of the wellbore. Eventually, the cuttings or other solids form a wellbore “skin” along the interface between the wellbore and the formation. The wellbore skin restricts the flow of the formation fluid and thereby damages the well.
In conventional (overbalanced) drilling conditions, the drilling fluid penetrates the reservoir, damaging the near-bore formation and obstructing the flow of oil and gas into the wellbore. This formation damage limits the productivity of the well. The less oil and gas an operator recovers from a well, the less money returned on their investment. Several years ago, one major operator estimated the net potential cost of formation damage over the remaining life of all of their fields at $1.5 billion before taxes.
Underbalanced drilling operations lighten the hydrostatic pressure of the drilling fluid column so that the pressure in the wellbore is less than the formation pressure at all times. The lower pressure in the wellbore encourages the oil/gas to flow from the formation and virtually eliminates the flow of drilling fluids into the formation. This increases the reservoir's rate of production and maximizes the recovery of available reserves.
The industry uses a dimensionless number called the skin factor to measure the amount of formation damage. The skin factor represents the degree which a wellbore is lined with particulate matter. The skin factor is proportional to the steady state pressure difference around the wellbore. The skin factor is calculated to determine the production efficiency of a wellbore by comparing actual conditions with theoretical or ideal conditions. Over three years, the production value of a well with a skin factor of ten might be $60 million. If the same well were drilled underbalanced-leaving it with a skin factor of two—the production value would typically be 75 percent higher or $105 million over the same three-year period.
The costs for underbalanced drilling (UBD) are higher than the costs for overbalanced drilling. Taken alone however, when benefits directly attributable to underbalanced drilling are considered, such as increased rates of penetration (ROP) and more trouble-free rig time, underbalanced drilling proves to be the more cost-effective drilling method. Lighter drilling fluids mean faster drilling time. Faster drilling time means lower drilling costs. Underbalanced drilling has been proven to increase the ROP by 100-500 percent. For example, an operator in Venezuela estimated drilling time for a conventionally drilled well at 43 days. The well was later drilled underbalanced in 17 days.
A lost circulation zone can drive up the cost of any well. It results in lost fluid, the addition of lost circulation materials, slower drilling time, and the reconditioning of the drilling mud when the zone is passed through—all additional costs. If the lost circulation zone causes the pipe to stick, then the costs of the equipment lost in the hole, fishing operations, sidetracking, and rig downtime will also be incurred. Underbalanced drilling provides insurance against such drilling problems because the pressure in the annulus is never greater than the formation pressure, and therefore, the pressure differential neither pushes the drilling fluid into the reservoir nor draws the pipe to the formation.
For example, a conventionally drilled well in Wyoming suffered fluid losses of 40,000 barrels as well as differential sticking (the well was sidetracked three times). The budget overrun was $6 million. By comparison, an underbalanced well was drilled in the pay section, experiencing total fluid losses of only 200 barrels and no differential sticking. The well was drilled under budget.
Underbalanced drilling can also curtail expensive stimulation costs. Stimulations are usually conducted to get beyond formation damage or to create artificial permeability in low-permeability zones. Since underbalanced techniques decrease the amount of formation damage and encourage the oil and gas to flow from the reservoir, underbalanced drilling can reduce or eliminate the need of stimulation.
Formulas for calculating skin factor based on geological data, experience, core samples, etc., are well known in the art. Companies have also modified these formulas or formulated new ones based on experience which they most certainly regard as proprietary. Once the skin factor is calculated, a production curve can then be calculated. Combining the production curve with cost data will yield the net present value (NPV) of the well. Compounding this, though, is the fact that a lot of the factors that go into calculating the skin factor and the costs are fraught with substantial uncertainty. Thus, the uncertainty associated with the skin factor and costs calculations must be statistically analyzed or “risked”, calculating skin factor and cost while varying the “riskable” parameters. Further, all of these calculations must be performed with all of the available completion methods, i.e., underbalanced and overbalanced completion, to enable selection of the best method.
Computer programs for performing at least some of these functions are also known in the art. However, performing all of these functions together involves splicing together numerous different computer programs and/or manual calculations, wasting valuable manpower. Thus, there is a need for a comprehensive computer program that allows a user to input all of the necessary data to perform rigorous skin factor calculations, cost analysis, flow projections, NPV analysis, and risking of all values associated with substantial uncertainty. Further, due to the uncertainty associated with many of the calculations, calibration of the software using data from existing wells would be very beneficial.